getopts

Check if an option is set

$ getopts [x] [opt]; echo $opt

$ getopts [x]: [opt]; echo $OPTARG

$ while getopts [xyz] [opt]; do case $opt in x) echo x is set;; y) echo y is set;; z) echo z is set;; esac; done

$ while getopts :[x:] [opt]; do case $opt in x) ;; :) echo "Argument required";; ?) echo "Invalid argument" esac;; done

$ OPTIND=1

getopts optstring name [args...]

optstring
    A string containing the option characters that getopts should recognize. If an option character is followed by a colon (:), the option requires an argument. If an option should be ignored put '+' symbol in front of the option character

name
    The name of the shell variable to set with the option character found.

[args...]
    Optional arguments to be parsed. If omitted, the positional parameters of the shell script are used.

The getopts utility is a shell builtin used to parse command-line arguments. It's primarily designed for use within shell scripts to handle options passed to the script. getopts iterates through the arguments, extracting and setting shell variables based on the option strings provided. This allows a shell script to flexibly respond to different command-line flags and arguments. Unlike more sophisticated option parsing tools, getopts handles basic option parsing well, it only supports short options (single character options) and optional arguments and may require manual parsing of positional parameters following the options.
It is useful when scripting simple scripts which don't require too much of complexity in options provided

getopts only handles short options (single characters). Long options (e.g., --verbose) are not supported directly. Positional arguments after the options must be handled separately. The parsing stops at the first non-option argument.

getopts returns 0 if an option was successfully parsed. It returns a non-zero value when it encounters the end of the options (indicated by '--') or an error. If an option requires an argument, and no argument is provided, getopts sets name to '?' and OPTARG to the name of the option.

OPTIND: Automatically updated to the index of the next argument to be processed.
OPTARG: Set to the argument of an option that requires one.

```bash
while getopts "ab:c" opt;
do
case "$opt" in
a) echo "Option a";;
b) echo "Option b with argument $OPTARG";;
c) echo "Option c";;
\?) echo "Invalid option: -$OPTARG" >&2;;
:) echo "Option -$OPTARG requires an argument." >&2;;
esac
done
shift $((OPTIND-1))
# Remaining positional arguments are now in $*```

getopts has been a standard shell builtin for a long time, appearing in various Unix-like operating systems and standardized by POSIX. It predates more sophisticated command-line argument parsing libraries and tools found in languages like Python or Perl, serving as a basic but reliable way to handle options in shell scripts.

getopt(1), argparse(1)